My present invention relates to a method of producing hollow extrusions using the cold nozzle process and, more particularly, to a method of producing a hollow extrusion of a thermoplastic synthetic resin which has a constant cross section over its length. The invention also relates to an apparatus for that purpose.
In the cold-nozzle process of thermoplastic synthetic resin extrusion, a melt of the synthetic resin is extruded through a heated die zone and directly thereafter and while under the extrusion-press pressure, is forced through a cooled calibration zone with internal and external cooling of the hollow strand.
The apparatus for carrying out that process generally comprises a heated die and a coolable calibration tool having a coolable mandrel arrangement forming the cavity or hollow of the extrusion, the die and the calibration tool being connected through the intermediary of a thermal insulating layer in a closed system which can be maintained under the extrusion pressure or the pressure of the melt during extrusion.
In the patent publication WO 96/30188, the thermoplastic synthetic resin melt in its final hollow chamber cross sectional shape passes from the die to the calibration tool. The mandrel arrangement which is of controllable temperature, i.e. can be heated or cooled, is held in the die and extends into the calibrator. If the hollow extrusion shape is to be maintained with minimal tolerances, it is necessary to operate with very low extrusion speeds with such apparatus. This is also the case when the hollow extruded shape has wall thicknesses which differ significantly from one another at different parts of the final cross section.
It is the principal object of the present invention to provide an improved method of producing a hollow extrusion, especially a hollow extrusion of a thermoplastic synthetic resin with a constant cross section over its length and with high precision at high speed.
Another object of this invention is to provide an improved apparatus for producing a hollow extrusion of complex configuration at high speed with the cold-nozzle process as described above.
It is also an object of this invention to provide a method of and an apparatus for producing hollow extrusions of thermoplastic synthetic resin whereby drawbacks of earlier approaches are avoided.
The reference to a xe2x80x9chollow extrusionxe2x80x9d here is intended to describe an extrusion of thermoplastic synthetic resin having at least one peripherally-closed chamber. Such extrusions generally have one or more flanges, wings or ribs extending from the peripherally-closed chamber and the peripherally-closed chamber can be of any cross sectional shape and usually is rectangular, trapezoidal or of another polygonal shape.
The extrusions can be used for a variety of purposes, for example in or as window frames, as structural framing members and as supports for panels or the like.
The aforementioned objects are achieved, in accordance with the invention, by extruding the synthetic resin melt in the form of a plurality of mutually independent open partial profiles which are oriented with respect to one another in accordance with the orientation of the segments of the finished hollow extrusion which are to be formed by these partial shapes. The webs of synthetic resin material between these partial extrusions or profiles, formed by an afterflow of the melt under the extrusion pressure which comes into contact with the partial profiles in the calibration zone so that at least in part by the heat of the melt, there is a fusion of the material of these partial webs to the partial profiles and a unitary product is formed. More particularly, the method of producing the hollow extrusion of thermoplastic synthetic resin of a constant hollow extrusion cross section over the length thereof comprises the steps of
(a) extruding a molten thermoplastic synthetic resin in a plurality of independent open partial-profile strands through a heated die zone in a pattern corresponding to cross-sectionally open segments of the hollow-extrusion cross section but with partial ribs having spaced-apart ends juxtaposed with one another across respective gaps into a cooled calibration zone, the strands being positioned and oriented correspondingly to the respective positions and orientations of the segments in the hollow-extrusion cross section;
(b) cooling the strands along inner and outer sides of the hollow-extrusion cross section while advancing the strands through the cooled calibration zone under extrusion-press pressure; and
(c) in the cooled calibration zone and downstream from the heated die zone feeding an afterflow of molten synthetic resin under the extrusion-press pressure and maintained flowable to this point into the gaps for fusion to the ends of the partial ribs under heat of the molten synthetic resin thereby forming webs bridging the partial ribs and completing the hollow-extrusion cross section.
Preferably at least in the regions in which the webs are extruded into contact with the ends of the partial ribs and where the molten synthetic resin of the webs is to be fused to the partial ribs, the corresponding portions of the periphery of the hollow shape in the calibration zone retain heat, thereby promoting the bond or fusion between the webs and the partial ribs under all conditions. The internal cooling of the hollow extrusion cross section can be effected first in the calibration zone and not, as is conventional, already begun in the die zone.
According to a feature of the invention, therefore, the method comprises the step of maintaining an elevated temperature of the strands at least in a region of its ends over a starting portion of the cooled calibration zone. The apparatus for producing the hollow extrusion can comprise:
means forming a heated extrusion die for extruding a molten thermoplastic synthetic resin in a plurality of independent open partial-profile strands in a pattern corresponding to cross-sectionally open segments of the hollow-extrusion cross section but with partial ribs having spaced-apart ends juxtaposed with one another across respective gaps, the strands being positioned and oriented correspondingly to the respective positions and orientations of the segments in the hollow-extrusion cross section;
a calibrator separated from the heated extrusion die by an insulating layer and forming an assembly therewith for advance of extrusion under extrusion-press pressure, the calibrator receiving the strands for cooling the strands along inner and outer sides of the hollow-extrusion cross section while advancing the strands through the cooled calibration zone under the extrusion-press pressure; and
means in the cooled calibration zone and downstream from the heated die for feeding an afterflow of still molten synthetic resin under the extrusion-press pressure into the gaps for fusion to the ends of the partial ribs under heat of the molten synthetic resin thereby forming webs bridging the partial ribs and completing the hollow-extrusion cross section.
For this purpose, respective channels can be formed in the extrusion die to produce the strands and the die can have a duct for supplying the molten synthetic resin which is branched to those channels.
According to another feature of the invention the calibrator is formed with respective channels receiving the strands, the apparatus further comprising thermally conductive projections extending into the channels from the extrusion die for maintaining an elevated temperature of the strands at least in a region of the ends over a starting portion of the cooled calibration zone. The projections can be provided in pairs on a heat-conducting insert and a mandrel can be retained within the insert and the respective projections.
The invention thus solves the problem of high extrusion speeds by forming segments of the hollow extrusion cross section as open strands, i.e. strands which are not peripherally closed, and thus which can be extruded at high speeds. Only in the calibration zone, after cooling at least initially, by merger with the afterflow of the synthetic resin melt under press pressure and maintained molten by the heat conductivity insert are these segments joined together to complete the hollow chamber configuration, i.e. to form the chamber with closed periphery.
From EP 0 584 467 it is known to produce segments of a hollow extrusion, but here, these segments after calibration, with heating anew, are joined together to form the hollow extrusion. The strands passing simultaneously from the die zone form the sum of the final hollow extrusion and the strands must be subjected to renewed heating to allow them to be pressed together. The method of the invention differs by conveying the thermoplastic melt into the cooled calibration zone and joining that melt with the individual strands so that heat from the melt effects fusion of the webs formed by the melt with the strands.